1. Field of the Invention
The present invention relates to a device and a method for correcting distortion and a computer-readable recording medium recorded thereon a distortion correction program. In particular, the invention relates to a distortion correction device and a distortion correction method for imaging an original document occupying two facing pages of an opened book or the like to reproduce the document as a plane image thereof, and relates to a computer-readable recording medium recorded thereon a program for operating a computer as a distortion correction device.
2. Description of the Related Art
When an original document on two facing pages of a bound book or the like is imaged from above, a resultant image is distorted due to the nonuniform height from a plate on which the document is placed (hereinafter referred to as “platen”) to the surface (to be imaged) of the document. FIGS. 21, 21A and 21B show an exemplary distorted image of an original document on two facing pages of an opened book (the document in this state is hereinafter referred to as “double page spread document”) that is imaged from above.
Referring to FIGS. 21, 21A and 21B, the image produced by imaging a double page spread document 2 has distortion of character lines as found in a circled region a (FIG. 21A) where characters are rotated differently depending on positions on the image. Moreover, character size differs depending on positions on the image as shown in a circled region b (FIG. 21B) where characters are further deformed. It is thus difficult to recognize characters from the distorted image. This problem of distortion is found not only in the character region but also in any photo regions and any regions enclosed by ruled lines.
The image distortion is briefly described below in conjunction with FIGS. 22A, 22B and 23 separately for distortions in the vertical and horizontal directions of an image of an original document.
Distortion occurring in the vertical direction is described first with reference to FIG. 22A which schematically shows double page spread document 2 viewed from one side. An imaging unit is positioned at point P1 above nearly the center (point P0) of double page spread document 2. Points A and B are at different heights respectively from the platen and accordingly apart from point P1 of the imaging unit by different vertical distances respectively.
FIG. 22B shows an image of the document having its portions of different sizes depending on horizontal positions on the image.
Different vertical distances from the imaging unit cause image size to vary. Specifically, as shown by solid lines in FIG. 22B, an image portion at a shorter vertical distance has a greater size while an image portion at a longer vertical distance has a smaller size. Then, depending on the position in the horizontal direction of the document, i.e., depending on the height from the platen, image portions have different sizes and thus the image portions differ from each other in terms of resolution.
Horizontal distortion is described next with reference to FIG. 23 which shows expansion and contraction of an original document occurring in the horizontal direction of the document. The document is imaged under the same condition as that shown in FIGS. 22A and 22B, and FIG. 23 also schematically shows double page spread document 2 viewed from one side. The imaged surface of double page spread document 2 is curved. Accordingly, portions on double page spread document 2 at even intervals are converted into image portions at uneven intervals on a plane as shown by FIGS. 24A and 24B. The resultant image thus has expanded and contracted portions in the horizontal direction.
A document which should originally be imaged as a plane image (FIG. 24A) is thus imaged as an image distorted in the horizontal and vertical directions (FIG. 24B).
Various methods have been proposed for correcting such image distortion hereinafter referred to as “book distortion”). For example, U.S. Pat. Nos. 5,362,958 and 5,652,805 disclose techniques of correcting book distortion by using spot light and slit light respectively to detect a surface shape of an original document. Japanese Patent Laying-Open No. 9-102854 discloses a technique of correcting book distortion by using a mirror which is set beside an original document to detect variation of height of the document viewed from the side thereof. Further, Japanese Patent Laying-Open No. 2000-20682 discloses a technique of making corrections by calculating variation in the height direction of an original document according to the state of distortion of an end of the document.
The correction methods as described above have following problems.
Specifically, the technique disclosed in U.S. Pat. No. 5,362,958 requires measurements of numerous positions for enhancing precision in detection of the surface shape of the document, namely precision in detection of height. Moreover, a sufficient amount of spot light must be secured for taking accurate measurements. A resultant problem is thus an increased device size and a longer processing time. In addition, a correction process is required for removing the influence of the spot light from the image in order to produce a final document image. Similarly, the technique disclosed in U.S. Pat. No. 5,652,805 has a problem in terms of the amount of slit light and requirement of correction process for eliminating the influence of the slit light.
The technique shown in Japanese Patent Laying-Open No. 9-102854 has a problem that an imaging unit and an object to be imaged are limited in position because the set mirror is used for detecting the height.
The technique shown in Japanese Patent Laying-Open No. 2000-20682 is intended to be applied to a contact type scanner for a fixed original image and a fixed imaging unit and thus is difficult to be applied to a hand-held type imaging unit or the like for an original document to be imaged and an imaging unit at respective relative positions that are not constant.
Namely, a problem resulting from the inconstant relative positions is that it is impossible to distinguish an end shape of a document having a remarkably varying height, the document viewed from almost directly above, from an end shape of a document having a moderately changing height, the document viewed in an angled direction. Then, variation in the height of the document cannot be specified resulting in an inaccurate correction.